Democritus (c. 460 - 370 B.C.), sometimes known as the "Laughing Philosopher", was a Pre-SocraticGreek philosopher from Thrace in northern Greece. Along with his teacher, Leucippus, he was the founder of the Greek philosophical school of Atomism and developed a Materialist account of the natural world.

Democritus was born in Abdera, a town in Thrace in northern Greece, which had originally been settled by Greek colonists from the Ionian city of Teos in present-day Turkey). His date of birth is usually given as 460 B.C., although some authorities argue for upto ten years earlier, and some for a few years later.

His father was very wealthy, and had even received the Persian king Xerxes on his march through Abdera. According to some accounts, Democritus studied astronomy and theology from some of the magi (wise men) Xerxes left in Abdera in gratitude.

On his father's death, Democritus spent his inheritance on extensive travels to distant countries, to satisfy his thirst for knowledge. He is reputed to have travelled to Persia, Babylon (modern-day Iraq), Asia (as far as India), Ethopia and Egypt (where he lived for five years, being particularly impressed by the Egyptian mathematicians). He also travelled throughout Greece to acquire a knowledge of its culture and meet Greek philosophers (he may have met the physician Hippocrates (c. 460 B.C.) and Socrates, and possibly also Anaxagoras, whom he praises in his own work), and his wealth enabled him to purchase their writings. He was known as one of the most travelled scholars of his time.

On returning to his native land, (now with no means of subsistence), he settled with his brother Damosis, and occupied himself with natural philosophy and gave public lectures in order to pay his way. His greatest influence was certainly Leucippus, with whom he is credited as co-foundingAtomism. In around 440 B.C. or 430 B.C., Leucippus had founded a school at Abdera, and Democritus became his star pupil. There are no existing writings which can be positively attributed to Leucippus, and so it is virtually impossible to identify which ideas were unique to Democritus and which are Leucippus', or any views about which they disagreed.

From anecdotal evidence, Democritus was known for his disinterestedness, modesty and simplicity, and appeared to live solely for his studies, declining the public honours he was offered. One story has him deliberately blinding himself in order to be less disturbed in his pursuits, although it is more likely that he lost his sight in old age. He was always cheerful and ready to see the comical side of life, and he was affectionately known as the "Laughing Philosopher" (although some writers maintain that he laughed at the foolishness of other people and was also known as "The Mocker"). His knowledge of natural phenomena (such as diagnosing illnesses and predicting the weather) gave him the reputation of being something of a prophet or soothsayer.

It is believed that he died at the age of 90, in about 370 B.C., although some writers have him living to over a hundred years of age.

Diogenes Laertius, the 3rd Century historian of the early Greek philosophers, lists a large number of works by Democritus, covering Ethics, physics, mathematics, music and cosmology, including two works called the "Great World System" and the "Little World System". However, his works have survived only in secondhand reports, sometimes unreliable or conflicting. Much of the best evidence comes from Aristotle, who was perhaps the chief critic of Atomism, although he nevertheless praised Democritus for arguing from sound considerations, and considered Democrtius an important rival in natural philosophy.

Like many other Pre-Socratic philosophies, the Atomism of Leucippus and Democritus was largely a response to the unacceptable claim of Parmenides that change was impossible without something coming from nothing (which is itself impossible), and thus any perceived change or movement was merely illusory.

In the Atomist version, there are multiple unchanging material principles which constantly rearrange themselves in order to effect what we see as changes. These principles are very small, indivisible and indestructible building blocks known as atoms (from the Greek "atomos", meaning "uncuttable"). All of reality and all the objects in the universe are composed of different arrangements of these eternal atoms and an infinite void, in which they form different combinations and shapes.

There is no room in this theory for the concept of a God, and essentially Atomism is a type of Materialism or Physicalism, as well as being atheistic and deterministic in its outlook. However, Democritus did allow for the existence of the human soul, which he saw as composed of a special kind of spherical atom, in constant motion, and he explained the senses in a similar manner.

In Epistemology, Democritus distinguished two types of knowledge: "bastard" (subjective and insufficient knowledge, obtained by perception through the senses), and "legitimate (genuine knowledge obtained by the processing of this unreliable “bastard” knowledge using inductive reasoning).

In the field of Ethics, Democritus pursued a type of early Hedonism or Epicureanism. He was one of the earliest thinkers to explicit posit a supreme good or goal, which he called cheerfulness or well-being (see the section on Eudaimonism) and identified with the untroubled enjoyment of life. He saw this as achievable through moderation in the pursuit of pleasure, through distinguishing useful pleasures from harmful ones, and through conforming to conventional morality. He is quoted as saying, "The brave man is he who overcomes not only his enemies but his pleasures".

Democritus was also a pioneer of mathematics and geometry, and produced works entitled "On Numbers", "On Geometrics", "On Tangencies", "On Mapping" and "On Irrationals", although these works have not survived. We do know that he was among the first to observe that a cone or pyramid has one-third the volume of a cylinder or prism respectively with the same base and height.

He was also the first philosopher we know who realized that the celestial body we call the Milky Way is actually formed from the light of distant stars, even though many later philosophers (including Aristotle) argued against this. He was also among the first to propose that the universe contains many worlds, some of which may be inhabited. He devoted many of the later years of his life to researches into the properties of minerals and plants, although we have no record of any conclusions he may have drawn.